The necessity for accurate alignment of coupled rotating shaft machinery is well known. More specifically, in order to assure maximum machinery longevity, accurate alignment between driving and driven components is essential. As is known in the art, if a coupling is set for perfect alignment in the cold or non-running condition, the machinery often drifts away from this orientation as operating temperatures change. This can lead to higher vibration levels, or harmful shaft misalignment, potentially resulting in premature excessive wear, or even in catastrophic failure.
In order to compensate for this occurrence, initial alignment of non-operating coupled shafts often includes an offset to account for relative anticipated thermal expansion or contraction as the machinery reaches full operating conditions. More specifically, the machinery is set out of alignment by a predetermined amount and allowed to grow into true alignment during operation. Often, however, accurate cold alignment offset figures are not available, forcing the field technician to attempt to align the rotating components without any guidelines for compensation. This inherently leads to inaccuracies and thence possibly to premature machine failure. This situation is unacceptable due to the related safety hazard, possible damage to the machine components and the related downtime.
A variety of shaft alignment/thermal growth measurement systems have attempted to address this problem. U.S. Pat. No. 4,102,052 to Bloch, discloses an apparatus for determining axial displacement or deflection of a rotating shaft or coupling due to temperature change, thus allowing compensation for shaft axial growth. A calibrated deflection decal is affixed to the coupling spacer tube and zeroed at a convenient point, such as at the guard of a standard diaphragm coupling. The machine is placed into service and axial deflection of the shaft is determined by reading the decal with the aid of speed synchronized stroboscopic light. While the use of this system has proved generally effective, it provides only data of sufficient precision to aid in correcting axial displacement. It does not address the problem of sufficiently correcting parallel and angular misalignment, and as such is only of limited effectiveness.
U.S. Pat. No. 4,428,126 to Banks discloses an apparatus for monitoring shaft alignment utilizing a bar or other mounting means attached to component housings. Eddy current proximity probes are utilized to obtain information that can be converted into shaft alignment change data. Eddy current proximity probes are also used in the system disclosed in U.S. Pat. No. 3,783,522 to Dodd. A more thorough use of such proximity probes is disclosed in U.S. Pat. No. 4,033,042 to Bently and U.S. Pat. No. 4,148,013 to Finn. The proximity probe systems disclosed in these references effectively measure running angular and parallel alignment of one machine relative to another to which it is flexibly coupled. These systems use shaft centerline relationships, and provide growth data to be used in calculating misalignment. While this is advantageous from the standpoint of allowing mathematical calculations, the systems are complex, expensive and difficult to retrofit to existing machines.
A different approach is disclosed in the applicant's U.S. Pat. No. 4,928,401, entitled Shaft Alignment System. A system is disclosed whereby vernier scales are attached to a machine coupling, and running alignment measurements are taken with speed synchronized stroboscopic light. The system measures hot or running angular and parallel alignment at the center of the machine shafts. This system incorporates many advantages over the others, but does exhibit limitations as to the types of machines and couplings to which it can be applied. Its use also requires that the machines be shut down prior to scale installation and again for scale removal following completion of measurement. On many machines that run continuously over long periods of time, this is inconvenient or impractical.
Various laser growth measurement and monitoring approaches have also been used. The laser alignment equipment, once properly mounted to adjacent machines, generally provides growth data that is then utilized to determine the appropriate offsets to be applied to the machine elements to bring about improved hot running alignment. U.S. Pat. No. 4,698,491 to Lysen discloses a laser alignment system whose primary purpose is for basic alignment of machine shafts while the machines are shut down. While the reference suggests attaching the laser equipment to the rotating shafts themselves, in most thermal growth measurement applications, somewhat different laser equipment is mounted to machine bearing housings. This advantageously allows continuous monitoring of shaft alignment. It further allows the equipment to be mounted while the coupled machines are running in anticipation of gathering data.
It can be appreciated that laser growth measurement data is preferable when taken from beginning to end of the hot-to-cold thermal cycle. This allows the machine operator to convert the growth data into offset figures and then apply the offsets to the non-running machines prior to start-up. This measurement procedure in usual applications heretofore has required the laser equipment to remain in place undisturbed throughout the cycle, which is difficult to accomplish with delicate instruments. It also becomes expensive if a multiplicity of machines are shut down in the same time frame, each requiring a set of laser equipment to accomplish simultaneous measurement.
A further limitation of this approach is the generally poor linearity of some laser targets over the range of thermal movement or growth encountered. More particularly, while laser equipment generally has excellent repeatability, i.e. bringing the laser beam axis and the laser target axis into substantial alignment (with the laser beam at the target center and perpendicular to the target surface), it has less than ideal linearity characteristics as the beam axis moves further away from target axis, representing some degree of thermal growth. Thus, as growth becomes greater, the prior art laser equipment becomes increasingly inaccurate. There is an inherent difficulty in compensating, calibrating or correcting for this poor linearity, although some systems achieve partial correction by algorithm or electronic means. In extreme cases, the relative movement of machines may cause the laser beam to move beyond the target aperture limits, thus losing the numerical display provided by equipment readouts and making attempted measurement completely unsuccessful.
A need exists, therefore, for a shaft alignment system for measuring thermal growth that incorporates the benefit of laser equipment repeatability while avoiding the drawbacks of its less than ideal linearity and target aperture size limits. Such a system would be simple to install and operate, and provide highly accurate results relating to both angular and parallel misalignment over a wide range of measurement. Further desirable features would include low cost, even with a multiplicity of machines shutting down in the same time frame, and capability of installation during machine operation, thus permitting more desirable hot to cold measurement. This would allow corrective adjustments to be made prior to resumption of machine operation. A further desirable feature would include the capability for easy testing of the system at any time for damage or deterioration, by plugging the components into a previously calibrated test stand assembly.